GitLab

Since https://crrev.com/c/2289856, TabModel stopped observing
WebState and _webStateObserver is no longer used. Remove the
ivar.

Bug: none
Change-Id: I1b3b73b69d3bcde06e598ce70a3074f095350869
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461117
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815227}